In a typical cellular radio system or a radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a radio access network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks (CN).
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Packet System (EPS) have completed within the 3GPP and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g., eNodeBs in LTE, and the core network. As such, the RAN of an EPS system has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
Recently two main trends have emerged in the cellular telephony business. First mobile broadband traffic is more or less exploding. The technical consequence is a corresponding steep increase of the interference in these networks, or equivalently, a steep increase of the load. This makes it important to exploit the load headroom that is left in the most efficient way. Secondly, radio communications networks are becoming more heterogeneous, with macro radio base stations being supported by micro radio base stations at traffic hot spots. Furthermore, home base stations, also called femto radio base stations, are emerging in many networks. This trend clearly puts increasing demands on inter-cell interference management.
Below it is described the measurement and estimation techniques, needed to measure the instantaneous total load, also referred to as the received total power value, on the uplink air interface.
WCDMA Load
The Need for Accurate Load Estimation
The air interface load of the WCDMA uplink is a fundamental quantity for                the scheduling, in the RBS, of enhanced uplink (EUL) users.        the admission and congestion control algorithms, in the RNC, that also control the load created by release 99 user equipments.        
There are several reasons for this. Firstly, the fast inner power control loop coupling between user equipments can create instability if too much load is allowed in the uplink, due to the so called the party effect. Such power rushes originate e.g. when a new high power user equipment is entering the uplink causing interference. Since the inner loop power control strives to maintain the signal to interference ratio (SIR) at a specified level, the consequence is that the other user equipments of the cell increase their powers, which in turn increases the interference and lead to additional power increases. At a certain point this process goes unstable with unlimited power increases of all user equipments in the uplink of the cell.
Secondly, it is well known that increased interference levels reduce the coverage, simply because a terminal or user equipment needs to transmit with a higher power to overcome an increased interference level. At the cell boundary, the UE power is hence saturated, meaning that the UE must move towards the base station to be detected—hence the cell size is reduced.
Thirdly, the scheduling of enhanced uplink user equipments in the RBS does not account for release 99 legacy traffic from UEs that do not support EUL. Even modern user equipments may lack support for EUL. In order to keep the air interface load under control, release 99 traffic must hence be monitored elsewhere. In WCDMA this control functionality is performed in the RNC, by the admission and congestion control algorithms. Since the consequences, instability and loss of coverage, are the same as for EUL user equipments when the air interface becomes over utilized, it follows that also the admission and congestion control algorithms need to have access to a measure of the momentary air interface load.
Finally, it is crucial that the load measure is accurate. This follows since the load, which is expressed as a (noise) Rise over Thermal, see below, is usually limited to be below 10-15 dB. It also follows that any load estimation errors will require margins that reduce the limit of 10-15 dB to lower values, a fact that will reduce the cell capacity. Hence all quantities that are used to form the uplink air-interface load need to be estimated very accurately, say at 0.1-0.2 dB level so as not to limit uplink mobile broadband performance.
The Need for Low Computational Complexity Load Estimation
For reasons described in section “Problems in WCDMA load estimation” below, load estimation in WCDMA is complicated and dedicated algorithms are needed in order to provide accurate estimates of both the thermal noise power floor, also referred to as the noise floor estimate, and the load in terms of the rise over thermal, also referred to as the noise rise estimate. Each of these algorithms is associated with a specific number of arithmetic operations per second, performed in a computer of some kind. The need for a low computational complexity is best understood by noting that:                The thermal noise power floor is unique for each analogue signal path, from the receive antenna in to the digital receiver unit. This is so because the thermal noise power floor measured in the digital receiver is dependent on variations due to at least i) component variations resulting in variation of the antenna element noise factor, ii) random variations of the component values to the front end electronics components that e.g. form pre-amplifiers, iii) variations due to installation of cabling and connectors at the RBS site. The consequence is that the thermal noise power floor needs to be estimated separately for each antenna branch, i.e. one thermal noise power floor estimation algorithm instance is needed for each antenna branch.        The rise over thermal is directly dependent on i) the thermal noise power floor of the antenna branch, and ii) the total received wideband power. Since i) introduces an antenna branch dependence, and since fading makes the received total wideband power vary significantly between antenna branches, it follows that also the rise over thermal floor needs to be estimated separately for each antenna branch, i.e. one rise over thermal estimation algorithm instance is needed for each antenna branch.        
The need for low complexity estimation algorithms is then due to the fact that many algorithm instances need to be executed in parallel in the RBS. To understand how many, it can be noted that an RBS supports:                Multiple sectors, typically three 120 degree sectors or six 60 degree sectors.        Multiple antenna branches per sector, today up to 4 antenna branches.        Multiple carriers, i.e. frequencies, per sector and antenna branch, today up to 2 carriers in the uplink.        
A typical RBS may have 3 sectors, 4 antenna branches per sector and 2 carriers per sector and antenna branch, i.e. a need for 3*4*2=24 algorithm instances for rise over thermal and thermal noise power floor estimation, respectively. Typical RBS configurations are subject to constraints between sectors, antenna branches and carriers. However, it can be foreseen that the numbers listed above will increase in the future, meaning that the total number of needed instances of the above two estimation algorithms may approach at least 100. The consequence is that it becomes crucial to minimize the computational complexity of each algorithm instance.
Problems in WCDMA Load Estimation
It is well known that the load at the antenna connector of the WCDMA uplink is given by the noise rise, or Rise over Thermal, RoT(t), defined by
                                          RoT            ⁡                          (              t              )                                =                                    RTWP              ⁡                              (                t                )                                                                    N                0                            ⁡                              (                t                )                                                    ,                            eq        .                                  ⁢                  (          1          )                    where N0(t) is the thermal noise power floor, also referred to as noise floor level, noise power floor level or thermal noise level, as measured at the antenna connector. t denotes the time. It remains to define what is meant with RTWP(t). The definition used here is simply the Received Total Wideband Power RTWP(t)
                                          RTWP            ⁡                          (              t              )                                =                                                    ∑                                  k                  =                  1                                K                            ⁢                                                P                  k                                ⁡                                  (                  t                  )                                                      +                                          I                N                            ⁡                              (                t                )                                      +                                          N                0                            ⁡                              (                t                )                                                    ,                            eq        .                                  ⁢                  (          2          )                    also measured at the antenna connector. Here IN(t) denotes the power as received from neighbour cells (N) of the WCDMA system, and Pk(t) is the power of the k:th user of the own cell. As will be seen below, the major difficulty of any RoT estimation algorithm is to separate the thermal noise power floor from the interference from neighbor cells.
Another specific problem that needs to be addressed is that the signal reference points are, by definition at the antenna connector. The measurements are however obtained after the analogue signal conditioning chain, in the digital receiver. The analogue signal conditioning chain does introduce a scale factor error, about 1-3 dB, 1-sigma, that is difficult to compensate for. Fortunately, all powers of eq. (2) are equally affected by the scale factor error so when eq. (1) is calculated, the scale factor error is cancelled as
                                          RoT            DigitalReceiver                    ⁡                      (            t            )                          =                                                            RTWP                DigitalReceiver                            ⁡                              (                t                )                                                                    N                DigitalReceiver                            ⁡                              (                t                )                                              =                                                                      γ                  ⁡                                      (                    t                    )                                                  ⁢                                                      RTWP                    Antenna                                    ⁡                                      (                    t                    )                                                                                                γ                  ⁡                                      (                    t                    )                                                  ⁢                                                      N                    Antenna                                    ⁡                                      (                    t                    )                                                                        =                                                            RoT                  Antenna                                ⁡                                  (                  t                  )                                            .                                                          eq        .                                  ⁢                  (          3          )                    
The superscripts Digital Receiver and Antenna indicate quantities valid at the digital receiver and the antenna respectively, and γ(t) denotes said scale factor error. In order to understand the fundamental problem of neighbor cell interference when performing load estimation, note thatIN(t)+N0(t)=E[IN(t)]+E[N0(t)]+ΔIN(t)+ΔN0(t)  eq. (4)
where E[ ] denotes mathematical expectation and where Δ denotes the variation around the mean. The fundamental problem can now be clearly seen. Since there are no measurements available in the RBS that are related to the neighbor cell interference, a linear filtering operation can at best estimate the sum E[IN(t)]+E[N0(t)]. This estimate cannot be used to deduce the value of E[N0(t)]. The situation is the same as when the sum of two numbers is available. Then there is no way to figure out the values of the individual numbers. This issue is analyzed rigorously for the RoT estimation problem in [1] where it is proved that the thermal noise power floor is not mathematically observable.
Currently, the radio subsystem of the WCDMA RBS is experiencing severe Central Processing Unit (CPU) overload. This overload is causing crashes and slow response to commands. The problem with existing algorithms for thermal noise power floor estimation and rise over thermal estimation is that the computational complexity is too high, when new functionality is introduced and when the number of algorithm instances increases.
In more detail:                The computational complexity of the fundamental rise over thermal (RoT) estimation algorithm, operating in the WCDMA uplink is too high. With up to 8 instances running in parallel on one radio subsystem, average CPU load is up above 90%, max target for healthy radio sub-system operation is below 70%. Here the RoT estimation is responsible for about 70% of the CPU load.        The computational complexity of the fundamental thermal noise power floor (N0(t)) estimation algorithm, operating in the WCDMA uplink is too high. These algorithm instances are executed much more seldom, but when they are executed, they cause additional load that makes the requested total CPU load exceed 100%. This may cause radio subsystem crashes. This is not acceptable since it affects key performance indicators, e.g. the average cell down time.        
In the future, it is planned to move said estimation algorithms to other parts of the RBS. Also then it is crucial to reduce the computational complexity, the reason being that after a move several hundred instances of said algorithms may need to be executed in parallel since there are several radio sub-system units for each RBS, and that the size of the RBS increases, e.g. by addition of antenna branches.